Resistor for thermogauges



Aug. 30, 1949. K. scHwARTzwALDER ETAL 2,480,166

RESISTOR FOR THERMOGAUGES Filed Jan. 8, 1945 Inventors Patented Aug. 3o,1949 2,480,166

UNITED STATES PATENT RESISTOR FOB, THERMOGAUGES Karl Schwartzwalder,Holly, and Alexander S.

Rulka and Robert W. Smith, Flint, Mich., assignors to General MotorsCorporation, Detroit,

Mich., a corporation of Delaware y Application January 8, 1945, SerialNo. 571,812

(Cl. 20L-76) Claims. l This invention relates generally to a thermogaugeor temperature indicator and more particularly to the composition andmanufacture of an improved resistor which varies in resistance orelectric conductivity with temperature changes to operate an indicatinginstrument such as a differential galvanometer.

Although shown herein as employed in an indicator system for thetemperature of the water fil battery 24 having a ground connection `26to the vehicle frame.

As shown in Figure 2,the unit I6 includes an adapter shell 28 which isthreaded into an opening in a Wall of the engine head il and supports aninwardly extending metal sleeve 30 having a closed inner end 32 and anopen outer end with a flange 34 to engage the shell 28. The resistor 36,which may be in the form of a rod cooling system of the internalcombustion enl0 having flattened sides as shown in Figure 3, is gine ofan automotive vehicle, it will be apparent mounted Within the Sleeve 30with e Spring 38 that the resistor is adapted for use in other engagingthe outer end of the resistor and holdtypes of temperature indicatorsystems or any ing the inner end of the resistor in contact withelectrical circuits wherein a predetermined varithe sleeve end 32. Boththe spring 33 and Sides ation in electricl conductivity is desired inre- 0f the leSiStOr 35 are insulated IOm iShe Sleeve sponse totemperature changes, by means of insulation 40 such as sh paper.According to the present invention, the re The spring 38 is interposedbetween the outer sistor is in the nature of a ceramic oxide which end0f the resistor 3s and the inner nd 0f a may be composed primarily ofmagnete and contact stud 42 which is threaded at its outer ferrie oxidein combination with glass phase that 20 end to recewe securing meanssuch as the nuts acts as a med bond M for the end of the conductor I8.The stud The preferred composition With certain modi- 42 1s Insulatedfrom trae dapter she? 28 md cations thereof and the method ofmanufacturle ange 34 of 'SI.eEVe y means o .was ers and 48 of Bakeliteor similar insulating mateing the resistor to he described hereinafterhave n 1 d l. k t 5u f bb th rk been found to produce a resistor whichsatises "5 ma an a' Sea mg gas e o ru er or e 1 e` such desired physicalrequirements as arelatively The outer Washer 48 may be pressed over aknurled portion of the stud into contact with a large and substantiallyuniform temperature .coshoulder 52 on the stud and the several partsemcient of resistance, electrical and chemical of the unit I6 may beheld together by turning stability, 'no appreciable change 1n resistanceon 30 over a flange 54 at the outer end of the Shell changes in appliedvoltage, arelatively low re- 2s in the manner shown in Figure 2SP'StIVlty to url lmlze se1 fhea'tmgf a9@ nodlrec' From the foregoingdescription it will be seen tional eiects 1n electrical conductivity 1nthe that one end 0f the resistor 36 will be grounded range of voltagesand currents encountered in through the 31181130 and adapter 23 to theenpractlcegine head I4 whereas the opposite end of the In i"he dlaWmgSresistor is electrically connected through the Figure 1 is adiagrammatic view of a part of an spring 38, stud 42 and conductor I8 tothe indiauiOmObile ShOWiIlg an engine temperature indicator 20. The heatfrom the water cooling syscator system. tem of the engine will betransmitted through the Figure 2 is an enlarged sectional view showing4,) shell 30 to the resistor 3s which, as will be dethe mounting of theresistor of the present inscribed hereinafter, has the characteristic ofvention. changing its electrical conductivity with a change Figure 3 isa perspective View of the resistor. in temperature, and the differentialgalvanome- Referring ISii t0 Figure 1, there has been ter 20 will becalibrated, as known in the art, shown a part of a conventionalautomotive vehito indicate the temperature of the water cooling cleincluding the usual body 4 with the windsystem in response to the amountof current flow. shield 6, the hood 8, and the instrument panelReferring now to the composition of the relil. The engine block l2 isprovided with the sistor 3E and the manufacture thereof which isY headit in which is installed a thermostatic unit the subject matter of thepresent invention, the llt for mounting the resistor of the presentinvenresistor consists generally of a ceramic oxide comtion which isconnected by the conductor I8 posed primarily of magnetite and ferrieoxide in' to a temperature indicator 20 such as a differcombination withglass phase that acts as a red ential galvanometer of known typeWhich'may bond. The composition which has been found to be connected inaccordance with conventional be most suitable for the use describedhereabove practice through the ignition switch 22 to the 55 consists ofapproximately 71.1% of magnetite (FeJOt), 8.9% of ferric oxide (FezOs),10% borosilicate glass, and 10% magnesium borate glass.`

In the foregoing composition the borosilicate glass may be that known asPyrex and the magnesium borate glass may be made from. 95% B20: and 5%MgO, but it should be understood that the glasses can vary considerablyin composition and the invention is not limited to use of theseparticular compositions.

It has also been found through actual practice that the preferredcomposition specified above can be varied as to both glass content andmagnetite and ferric oxide contents. Addition of other metallic oxidessuch as chrome oxide, manganese oxide, nickel oxide, etc., have beenmade but for the present purpose, the above composition has thus farbeen the most suitable. In all cases some glass as a bond has been foundto be desirable.

In the manufacture of the resistor, the constituents are first milledtogether for about one' half hour, preferably in a porcelain lined millwith porcelain balls. Then about 8% of a binder such as Dritcx, which isa hydrogenated oil, is added and the mixture ground for about fifteenminutes.

The mixture is then removed from the mill, admixed with water, andgranulated through a 20-mesh screen. It is dried in a steam drier andpreferably again granulated through a further screen such as 14-meshsince when passed through the .20-mesh screen in a Wet condition thelength of some parts may be greater than 20- mesh and the mixture alsoswells to some extent after being pressed through this first screen.

The granulated mixture is then pressed in metal dies into the desiredshape and form, such as the rod of Figure 3, and placed in a furnace ata temperature of about 700 F. for thirty minutes to burn out the Dritex.It has been found desirable to put the formed rods into stainless steeltrays covered with calcined alumina during this heating operation andthe alumina cleaned from the rod surface after cooling. The rods arethen heated in a nitrogen atmosphere to about 1600 F. in a continuousbelt type furnace on a forty-three minute cycle to melt the glasses andbond the materials together.

The ends of the rods are coated with copper, as indicated at 56 inFigure 3, in any known manner such as by spraying, and the resistancechecked at 200 F. The rods may then be copper sprayed or coated in someother known manner to give the resistance desired for the particularapplication. In the present instance the resistor is formed into astrong, hard and relatively nonporous rod about one inch long andone-quarter inch in diameter which has the following properties.

Its resistivity at 25 C. is approximately 27 ohm-cm and its temperaturecoeicient of resistance is 3% per degree centigrade. This lowresistivity minimizes self-heating and allows the resistor to carryhigher currents without excessive self-heating. The resistor ischemically stable and unaffected by contact with ethylene glycol,alcohol, oil or grease. It is slightly soluble in water and must beprotected when used under conditions of vhigh humidity or where there isdanger of contact with Water or steam. For applications, however, whereit is not feasible or desirable to avoid contact with water it appearsfrom preliminary experiments that the eliminaoxide, 10%

tion of the magnesium borate glass and use of only Pyrex as the glassphase in our composition may provide a resistor whose stability is notaffected by humidity.

The electro-resistive effect is almost entirely absent in this resistorover the range of voltages encountered in practice, and for the rangerelevant to the present application, such as from 4 to 24 volts thechange of resistance with voltage is negligible. The electricalconductivity also exhibits no directional effects in the range ofvoltages and currents encountered in practice and the resistor may beused in D. C. or A. C. circuits interchangeably without specialprecautions because of volume rectification effects. Since the resistoris formed in a slightly oxidizing atmosphere at a temperature muchhigher than that encountered over its operating range, it iselectrically stable and requires no pre-aging or protection from directcontact with air or other oxidizing atmospheres.

It will be apparent that certain variations from the preferred method ofmanufacture described herein may be made by those skilled in the art, aswell as modifications in the composition of the resistor, within thescope of the present invention as defined in the appended claims.

We claim:

1. A thermally sensitive resistor consisting of approximately 70%magnetite, 10% ferric oxide, and the remainder glass phase.

2. A thermally sensitive resistor consisting of approximately 70%magnetite, 10% ferrie oxide, and a glass bond consisting ofapproximately equal amounts of borosilicate glass and magnesium borateglass.

3. A thermally sensitive resistor consisting of a bonded mixture of71.1% magnetite, 8.9% ferric borosilicate glass and 10% magnesium borateglass.

4. A thermally sensitive resistor comprising a glass bonded compositionconsisting of at least about 70% magnetite with a glass phase includingmagnesium borate glass.

. 5. A thermally sensitive resistor comprising a glass bondedcomposition having a substantially uniform temperature coefficient ofresistance consisting of at least about 70% magnetite with a glass phaseincluding borate glass.

KARL SCHWARTZWALDER. ALEXANDER S. RULKA. ROBERT W. SMITH.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 860,997 Steinmetz July 23, 19072,027,413 Andres Jan. 14, 1936 2,106,578 Schwartzwalder et al. Jan. 25,1938 2,152,286 Schoenmaker Mar. 28, 1939 FOREIGN PATENTS Number CountryDate 231,335 Switzerland June 1, 1944 OTHER REFERENCES Ser. No. 377,578,Bieeld (A. P. C.), pub. May 18, 1943.

